How to get soft shadows in Solid View?
Ensure your mouse is over your 3D view, and hit 'n' if your properties panel isn't already showing. Then, just click 'Ambient Occlusion'. This simulates only shaded areas. 

If you tick 'Matcap' above this, the combination of these options is very handy for checking the smoothness or subtle details in your mesh. Great for sculpting, or just checking for imperfections that need fixing.